Custom routing values can no longer be extracted from the document body, but
must be specified explicitly as part of the query string, or in the metadata
line in the bulk API. See Type meta-fields for an
example.

The default hash function that is used for routing has been changed from
djb2 to murmur3. This change should be transparent unless you relied on
very specific properties of djb2. This will help ensure a better balance of
the document counts between shards.

In addition, the following routing-related node settings have been deprecated:

cluster.routing.operation.hash.type

This was an undocumented setting that allowed to configure which hash function
to use for routing. murmur3 is now enforced on new indices.

cluster.routing.operation.use_type

This was an undocumented setting that allowed to take the _type of the
document into account when computing its shard (default: false). false is
now enforced on new indices.

The delete API used to be broadcast to all shards in the index which meant
that, when using custom routing, the routing parameter was optional. Now,
the delete request is forwarded only to the shard holding the document. If you
are using custom routing then you should specify the routing value when
deleting a document, just as is already required for the index, create,
and update APIs.

To make sure that you never forget a routing value, make routing required with
the following mapping:

Previously, meta-fields like _routing, _timestamp, etc would only be
included in a GET request if specifically requested with the fields
parameter. Now, all meta-fields which have stored values will be returned by
default. Additionally, they are now returned at the top level (along with
_index, _type, and _id) instead of in the fields element.

The replication parameter has been removed from all CRUD operations
(index, create, update, delete, bulk) as it interfered with the
synced flush feature. These operations are now
synchronous only and a request will only return once the changes have been
replicated to all active shards in the shard group.